National Aeronautics and Space Administration Science Teacher’s Background SciGALAXY Q&As NASA / Amazing Space Science Background: Galaxy Q&As 1. What is a galaxy? A galaxy is an enormous collection of a few million to several trillion stars, gas, and dust held together by gravity. Galaxies can be several thousand to hundreds of thousands of light-years across. 2. What is the name of our galaxy? The name of our galaxy is the Milky Way. Our Sun and all of the stars that you see at night belong to the Milky Way. When you go outside in the country on a dark night and look up, you will see a milky, misty-looking band stretching across the sky. When you look at this band, you are looking into the densest parts of the Milky Way — the “disk” and the “bulge.” The Milky Way is a spiral galaxy. (See Q7 for more on spiral galaxies.) 3. Where is Earth in the Milky Way galaxy? Our solar system is in one of the spiral arms of the Milky Way, called the Orion Arm, and is about two-thirds of the way from the center of the galaxy to the edge of the galaxy’s starlight. Earth is the third planet from the Sun in our solar system of eight planets. 4. What is the closest galaxy that is similar to our own galaxy, and how far away is it? The closest spiral galaxy is Andromeda, a galaxy much like our own Milky Way. It is 2. million light-years away from us. Andromeda is approaching our galaxy at a rate of 300,000 miles per hour. Five billion years from now it may even collide with our Milky Way galaxy. 5. Why do we study galaxies? By studying other galaxies, astronomers learn more about the Milky Way, the galaxy that contains our solar system. Answers to such questions as “Do all (… continued on page ) NASA / Amazing Space Science Background: Galaxy Q&As (… continued from page ) galaxies have the same shape?,” “Are all galaxies the same size?,” “Do they all have the same number of stars?,” and “How and when did galaxies form?” help astronomers learn about the history of the universe. Galaxies are visible at vast distances, and reveal the history of the visible universe with their collections of bil- lions of stars, gas, and dust. 6. What are the parts of a galaxy? A galaxy contains stars, gas, and dust. In a spiral galaxy like the Milky Way, the stars, gas, and dust are organized into a bulge, a halo, and a disk containing spiral arms. Elliptical galaxies have a bulge shape and a halo, but do not have a disk. • Bulge—A round structure made up primarily of old stars, gas, and dust. The bulge of the Milky Way is roughly 10,000 light-years across. The outer parts of the bulge are difficult to distinguish from the halo. • Disk—A flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The disk of the Milky Way is 100,000 light-years across and 2,000 light-years thick. It contains mostly young stars, gas, and dust, which are concentrated in the spiral arms. Some old stars are also present. • Spiral arms—Curved material of gas, dust, and young blue stars that begins at the bulge of a spiral galaxy and curves outward, giving a galaxy a “pinwheel” appearance. Spiral arms are found only in spiral galaxies. • Halo—A roughly spherical collection of old stars, clusters of old stars (called globular clusters), and a little bit of gas and dust that extends farther than all other components of a galaxy. Halos contain dark matter, which is material that we cannot see but whose gravitational force can be measured. Halos may be more than 130,000 light-years across. • Stars, gas, and dust—Stars come in a variety of types. Blue stars are very hot and have shorter lifetimes than cooler, red stars. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has (… continued on page ) 3 NASA / Amazing Space Science Background: Galaxy Q&As (… continued from page ) been no recent star formation. Spiral galaxies have lots of gas and dust, while elliptical galaxies have very little gas and dust. 7. How are galaxies classified? What do they look like? Galaxies can be classified according to their shapes: spiral, elliptical, or irregular. Edwin Hubble, for whom the Hubble Space Telescope is named, devised another famous classification scheme for galaxies. Hubble’s system included elliptical and spiral galaxies but excluded irregulars. Today, astronomers use three main galaxy classes: spirals, ellipticals, and irregulars: • A spiral galaxy consists of a flattened disk containing spiral (pinwheel-shaped) arms, a halo, and a bulge at its center. Spiral galaxies have a variety of shapes. They are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain many young blue stars and lots of gas and dust. Most of the stars in the bulge are older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. These galaxies rotate somewhat like a hurricane or a whirlpool. • An elliptical galaxy does not have a disk or spiral arms. It is characterized by a smooth, ball-shaped appearance. Ellipticals contain old stars and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). The smallest elliptical galaxies (called dwarf ellipticals) are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move in randomly oriented orbits within the galaxy, like a swarm of bees. • An irregular galaxy is neither a spiral nor an elliptical. Irregular galaxies tend to be smaller objects without a definite shape, and they typically have very hot newborn stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation. Sometimes their irregular shape is the result of interactions or collisions with other galaxies. Observations such as the (… continued on page ) NASA / Amazing Space Science Background: Galaxy Q&As (… continued from page ) Hubble Deep Fields show that irregular galaxies were more common in the distant (early) universe. 8. How do astronomers classify galaxies today? Hubble’s system included elliptical and spiral galaxies but excluded irregulars. Today, astronomers use three main galaxy classes: ellipticals, spirals, and irregulars. Elliptical galaxies range from round shapes (E0) to oval shapes (E7). Spiral galaxies have a pinwheel shape and are classified according to their bulge, as well as how tightly their arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms, to Sc, which has a small bulge and loose, lumpy arms. Barred spiral galaxies classified as SB are pinwheel-shaped and have a distinct “bar” of stars, dust, and gas across their bulge. They range from an SBa, which has a bar across its large bulge and tight, smooth arms, to an SBc, which has a bar across its small bulge and loose, lumpy arms. Irregular galaxies have no definite shape but still contain new stars, gas, and dust. The chart below summarizes the properties of the main classes of galaxies. Main Classes of Galaxies Spiral galaxies Elliptical galaxies Irregular galaxies • Huge; contain stars, • Huge; contain stars, • Huge; contain stars, gas, and dust gas, and dust gas, and dust • Held together by gravity • Held together by gravity • Held together by gravity • Pinwheel shape • Round-to-oval shape • No regular shape • Bulge and thin disk; • Bulge but no disk; halo • May show signs of a halo is present is present disk and/or a bulge; halo is present • Rich in gas and dust • Small amount of cool • Usually rich in gas and dust gas and dust • Young and old stars are • Mainly old stars are • Young and old stars are present present present NASA / Amazing Space Science Background: Galaxy Q&As 9. What is the most common type of galaxy in the nearby universe? When one counts both large and small galaxies, dwarf ellipticals (small ellipticals) are probably the most common type of galaxy in the nearby universe. Since these galaxies are small and faint, their exact number is not well known. The majority of large, bright galaxies in the nearby universe are spirals. 10. Galaxies are often identified by a group of letters and numbers. What do they stand for? Scientists classify galaxies in different catalogs. The most common catalog is NGC, which stands for The New General Catalogue of Nebulae and Clusters of Stars. Other catalogs include M (Messier), ESO (European Southern Observatory), IR (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog). Sometimes a galaxy appears in more than one catalog and can have more than one name. The numbers following the letters, such as Mrk 917 (Sc) or NGC 133 (SB), indicate the galaxy’s entry in the catalog and are often related to the galaxy’s relative position in the sky. 11. What are colliding galaxies? When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies then may pass by each other or collide. Two galaxies that are interacting or colliding may be referred to as a pair, or one galaxy may be referred to as a companion of the other.